Method for producing a tube of glass

ABSTRACT

A method for forming a hollow cylinder, in a single step or in as small a number of steps as possible, into a quartz glass tube with a large outer diameter and high dimensional stability is provided. The cylinder, while rotating about a rotation axis, is softened in portions in a heating zone which is moved at a relative feed rate Va, and the softened portion is radially expanded by a centrifugal force and/or an internal overpressure applied in the hollow cylinder bore so as to form a deformation zone. The tube is continuously shaped with an outer diameter D 2  which is greater than that of the hollow cylinder D 1 . The radial expansion of the softened portion is carried out at a location-dependent radial expansion rate Vr, the profile of which along the deformation zone has a maximum value Vr,max which is smaller than two times the feed rate Va.

BACKGROUND OF THE INVENTION

The present invention relates to a method for producing a tube of glass,particularly of quartz glass, by forming a hollow cylinder from theglass with an outer diameter D₁ in that the cylinder, while rotatingabout a rotation axis, is softened in portions in a heating zone whichis moved at a relative feed rate Va, and the softened portion isradially expanded under the action of a centrifugal force and/or of aninternal overpressure applied in the hollow cylinder bore so as to forma deformation zone, and the tube is continuously shaped with an outerdiameter D₂ which is greater than D₁.

With such methods and apparatuses, hollow cylinders of glass,particularly of quartz glass, are formed in one or plural hot formingsteps into tubes, the radial tube dimensions being changed with respectto the radial dimensions of the hollow cylinder or the cross-sectionalprofile. An initial hollow cylinder which is rotating about itslongitudinal axis is here softened zone by zone and is expanded in thisprocess—under the action of a radially outwardly directed force—eitheragainst a molding tool, which is arranged at a predetermined radialdistance relative to the longitudinal axis of the tube, or it is formedwithout tools. The radially outwardly directed force is based on thecentrifugal force and/or on an internal overpressure in the inner boreof the hollow cylinder (also called “blow pressure”).

Special attention is here paid to the dimensional stability of thedrawn-off tube strand. To ensure this stability, constant detection andcontinuous control of a radial dimension of the tube strand, such as theouter diameter, the inner diameter or the wall thickness, areindispensable. The blow pressure, the relative feed rate between hollowcylinder and heating zone, and the temperature in the heating zone arecommon as a control variable of such a control.

Dimensional deviations that already exist in the original hollowcylinder tend to propagate into the drawn-off glass tube during theforming process and are even intensified in this process. Variations inthe radial cross-sectional profile or wall one-sidedness; i.e. radiallyirregular profiles of the tube wall thickness, also called “siding”among the experts, are here particularly disadvantageously noticed.Since upon use of a molding tool, the outer diameter is a relativelyfixed given dimension, tube wall one-sidedness is here accompanied byvariations in the inner diameter of the tube.

These problems increase with an increasing end diameter of the tube, forwall thickness variations found in the start cylinder exponentially growwith the diameter in the forming process. Therefore, in the finalanalysis, the maximum values for siding that can still be toleratedaccording to the specification (e.g., 1 mm) limit the virtuallyachievable end diameter of the tube. This effect also depends on thelevel of the blow pressure, so that this pressure cannot be arbitrarilyhigh. Instead of this, in order to achieve commercially acceptableforming rates, the glass must be heated to a higher degree and softenedmore strongly. This, in turn, leads to more drawing streaks or otherdefects in the glass wall and to an increased energy demand, especiallyin the case of large-volume tubes (also called “large tubes”hereinafter) which cool down very rapidly because of their large volume.

The greater the end diameter of the tube, the more difficult and morecost-intensive is therefore the production of a dimensionally stablelarge tube. To mitigate this problem, it is suggested in JP 2004-149325A that the forming process should be subdivided into a plurality offorming stages with successive increase in the diameter. For thispurpose, the hollow cylinder of quartz glass to be formed, which has adiameter of 250 mm, is clamped in a lathe and is rotated about itshorizontally-oriented longitudinal axis while it is heated by means of aring-shaped arrangement of heating burners and is softened zone by zonein that the heating burners are moved at a predetermined feed rate Vaalong the cylinder surface. The increase in diameter is due to thecentrifugal force acting on the softened portion. The deformation zonewill migrate along the whole start cylinder once until the cylinder isfully expanded. The outer diameter of the tube is here continuouslycaptured by means of a laser beam without tools. This forming step willbe repeated until the nominal tube diameter of 440 mm is reached. Ineach forming step, the tube diameter is increased by 15 mm.

CN 102887626 A describes a multi-stage forming process for producing aquartz glass tube with an outer diameter of more than 520 mm withforming stages of 60 mm each.

In this forming process, one achieves a comparatively small formingdegree in each individual forming stage, which is accompanied by asmaller deviation from the nominal value of a radial tube dimension.Moreover, each forming stage offers the possibility of considering andcorrecting dimensional deviations found in the respective startcylinder.

On the other hand, it is evident that this procedure requires a lot oftime and energy, especially since the tube cools down between successiveforming steps.

The attempt can be made to keep the number of the forming steps as smallas possible in that the respective deformation degree, i.e. the changein diameter, is set as high as possible. It has, however, been foundthat the forming process in the case of very great diameter changesbecomes unstable, which first manifests itself in diameter variationsthat form a wave structure extending in the longitudinal axis direction.

BRIEF SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a method whichmakes it possible to form a hollow cylinder in a single forming step orin a number of forming steps, which number is as small as possible, intoa glass tube with a large outer diameter and high dimensional stability.

This objective is achieved according to the present invention in thatthe radial expansion of the softened portion is carried out at alocation-dependent radial expansion rate Vr, the profile of which, alongthe deformation zone, has a maximum value Vr,max which is smaller than 2times the feed rate Va.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown.

In the drawings:

FIG. 1 is a side view and a schematic representation of an apparatus forforming a hollow cylinder of quartz glass into a quartz glass tube,according to an embodiment of the present invention;

FIG. 2 is schematic representation of a section of the apparatus of FIG.1 with additional constructional details;

FIG. 3 is a graphical sketch demonstrating parameters of the formingprocess, according to an embodiment of the present invention;

FIG. 4 is a diagram demonstrating the influence of blow pressure andcentrifugal force on tangential forces in the wall of a tube during theforming process; and

FIG. 5 is a diagram demonstrating the dependence of the geometricalshape of the deformation zone on the power density input.

DETAILED DESCRIPTION OF THE INVENTION

In the deformation portion (here also called “deformation zone”), thediameter of the softened glass strand is continuously increasing fromthe hollow cylinder to the tube in response to the deformationresistance of the glass, which is specifically determined by wallthickness and viscosity. In a longitudinal section, the deformationportion has a funnel shape on the whole, with an S-shaped transition,when viewed in section, between cylinder and tube, which will also becalled “shoulder” hereinafter.

It has been found that the aforementioned process instability will beginand the wave structure will be formed whenever the deformation rate ofthe glass in radial direction (here also called “radial expansion rate”)becomes too great in comparison with the initial value at the beginningof the radial deformation. The radial expansion rate is proportional tothe ratio of the outer diameter of the tube to the wall thickness of thetube. This means that the radial expansion rate is the greater, thegreater the outer diameter of the tube and the smaller its wallthickness are. The outer diameter is increasing from the beginning tothe end of the deformation portion, and the wall thickness is normallydecreasing, but it may also remain approximately the same. That is why,without any counter-measures and at the same temperature, the radialexpansion rate is increasing from the beginning of the deformationportion to the end. At a constant relative rate between heating zone andhollow cylinder, this will have the outcome that the slope in the areaof the “shoulder” is getting steeper and steeper. This, however, reducesthe convective heat input into the glass from the radial direction,which effects a relative local cooling and thus a decrease in the localradial expansion rate. The “shoulder” thereby becomes flatter again andthe radial heat radiation input into the glass and the local temperaturebecome higher again, so that the local radial expansion rate isincreasing again and the mechanism starts anew. The system oscillates,thereby causing the circulating waves.

The basic idea for avoiding this process instability is now tocounteract the occurrence of an excessive radial expansion rate in thedeformation portion.

A high radial expansion rate Vr is harmless if the deformation zone isalso moving at a high rate; i.e. if the axial feed rate Va is high. Thefeed rate is normally kept constant throughout the forming process.Therefore, a measure of the degree of the radial expansion rate at agiven feed rate is the slope of the “shoulder” in the deformationregion. The shoulder is the steeper, the greater the ratio betweenradial expansion rate and axial feed rate. In other words, the slope ofthe shoulder follows from the ratio of the rates Vr/Va, wherein Vrrefers to the expansion of the radius; i.e. half the diameter.

It has been found that process fluctuations are avoided if this ratio issmaller than 2, or even better smaller than 1.5, preferably smaller than1, and particularly preferably smaller than 0.7. The maximum radialexpansion rate Vr,max is here preferably smaller than 20 cm/min,particularly preferably smaller than 10 cm/min.

The forming of the hollow cylinder is based on the blow pressure or onthe centrifugal force during rotation (centrifugation) or on botheffects. The tangential tension σT which is operative in the tube wallis composed of the following two constituents, of which the first onedescribes the effect of the blow pressure, and the second one the effectof the centrifugal force.σT=p·r/WD+ρω ² r ²  (1)where: p=pressure, r=radius in the forming portion, WD=wall thickness,ρ=specific density of the glass, ω=angular velocity.

The tangential tension which is operative in the tube wall (withoutcentrifugation; only blow pressure) produces the radial expansion in thedeformation zone, i.e.:

$\begin{matrix}{\sigma_{T} = {\eta_{D}\frac{\mathbb{d}ɛ}{\mathbb{d}t}\left( {{\eta_{D}\text{:}\mspace{14mu}{extensional}\mspace{14mu}{viscosity}},{ɛ \approx \frac{r - r_{0}}{r_{0}}}} \right)}} & (2)\end{matrix}$

Thus, the following applies to the expansion rate of the glass in radialdirection (and based on the radius):

$\begin{matrix}{\frac{\mathbb{d}ɛ}{\mathbb{d}t} = \frac{\sigma_{T}}{\eta_{D}}} & (3)\end{matrix}$

Thus, the viscosity of the glass is a parameter suited for varying thelocal deformation rate, because the deformation rate is inverselyproportional to the viscosity, and the latter depends on the temperatureof the glass. Thus, it is suggested as a suitable measure for thesolution of the problem and for varying the maximum value of the radialexpansion rate Vr, that the temperature profile acting on the tubestrand along the deformation portion should be adapted such that thedifference of the deformation rate in radial direction (of the radialexpansion rate) between beginning and end should be kept as small aspossible; ideally, a rise is completely compensated by the temperatureprofile.

It has been found to be advantageous, in this respect, when in theheating zone, a temperature profile is generated that along at least asub-length of the deformation zone has a course opposite to a profile ofthe tangential tension along the same sub-length.

Differences in the profile of the tangential tension along thesub-length, and thus differences in the radial expansion rate, are herecompensated completely or at least in part by the temperature profile,with the aim to set a radial expansion rate which is as constant aspossible along the sub-length. As a rule, this aim is reached all themore completely, the greater the corresponding sub-length of thedeformation zone is, in which the temperature profile and the profile ofthe tangential tension are opposite to each other. Ideally, thesub-length is therefore the whole length of the deformation zone.However, it is also enough to concentrate on the particularly criticalarea around the middle of the deformation zone; e.g. the middle third ofthe deformation zone. The temperature profile within the heating zoneis, for example, obtained on the basis of the nominal heating outputcurve during operation, or it is determined by measurement of thesurface temperature on the tube to be formed.

The calculation of an “inverse” temperature profile which completelycompensates the profile of the tangential tension is carried out on thebasis of the above equations (1) to (3).

The radial expansion rate Vr is proportional to the ratio tangentialtension/viscosity, i.e.:dε/dt=σ/η=2·π·V _(r)

For the tangential tension: σ_(T)=(p·r)/WD+ρω²r²

Viscosity and its temperature dependence are glass-specific. For theviscosity η of quartz glass, the following dependence η(T) isapplicable:lg=η=1.6+8487/(T[° C.]−390) (in Pas)

Thus, under the condition of a constant expansion rate Vr, one obtainsthe following for the “inverse” temperature profile:

$\begin{matrix}{T = {\frac{8487}{{\lg\left( \frac{\frac{p \cdot r}{WD} + {{\rho\omega}^{2}r^{2}}}{2 \cdot \pi \cdot V_{r}} \right)} - 1.6} + 390}} & (4)\end{matrix}$

In practice, a complete compensation of different radial expansion ratesis not required most of the time. The profiles of temperature andtangential tension are not required to be exact mirror images or to beinverse or opposite. In simple cases, the demand on the inversion of theprofiles is satisfied if in the case of a profile in which thetangential tension increases in the direction of the tube, thetemperature decreases in the same direction. Or in the case of a profilein which the tangential tension at an axial position along thesub-length of the deformation zone under consideration has a maximum,the temperature profile at this axial position has a minimum.

Due to a decreasing wall thickness, a particularly high radial expansionrate would have to be expected in the case of a homogenous temperatureprofile along the deformation zone at the end thereof. The sketch ofFIG. 3 schematically shows the area of the deformation zone 30 betweenhollow cylinder 2 and tube 22 in a diagram. Diameter D is here plottedon the ordinate, and the longitudinal axis position x on the abscissa.For the purpose of explaining the effect of undesired great differencesin the radial expansion rate, the hollow cylinder 2 which is rotatingabout the longitudinal axis 6 has a greater wall thickness than thedrawn-off tube 22, resulting in a reduction of the wall thickness alongthe deformation zone 30. The deformation zone 30 is here gray-shaded.

The “beginning” of the deformation zone 30 is defined as that x-positionat which the following applies to the location-dependent outer diameterDv of the deformation zone: D_(v1)=D1+ 1/10x(D2−D1). Thus the “end” ofthe deformation zone marks that x-position at which the followingapplies to the location-dependent outer diameter D_(V) of thedeformation zone: D_(v2)=D2− 1/10x(D2−D1).

In the front region of the deformation zone 30, with a comparativelylarge wall thickness WD, a radial expansion rate which is represented bythe vector v_(r1) is obtained on the basis of the above equations (1) to(3). By comparison, a comparatively higher radial expansion rate isobtained in the rear region of the deformation zone 30, with a thinnerwall thickness WD—at a constant feed rate v_(a), as indicated by thelonger vector v_(r2).

To counteract this, the temperature toward the end of the deformationzone is preferably lower than at the beginning of the deformation zone.The temperature difference between end and beginning of the deformationzone is here at least 20° C.

To generate a non-homogeneous temperature profile along the deformationzone, the heating zone is subdivided into two or more heating portionsthat are heatable independently of each other. In a particularlypreferred embodiment, it is intended that the heating zone has a frontheating portion assigned to the beginning of the deformation zone and arear heating portion assigned to the end of the deformation zone, with ahigher temperature being generated on the surface of the deformationzone by means of the front heating portion than by means of the rearheating portion.

Ideally, the radial expansion rate between the beginning (Dv1) and theend (Dv2) of the deformation region is constant. In practice, it hasbeen found to be useful that the radial expansion rates at the positionsdiffer by not more than 50% (based on the smaller one of the twovalues).

A further parameter for varying the local radial expansion rate is thewall thickness of the drawn-off glass strand. The thicker the tube wall,the greater is the deformation resistance to radial expansion at thesame viscosity. The radial expansion rate is inversely proportional tothe wall thickness. It is determined by the ratio of the speed at whichthe hollow cylinder passes into the heating zone and the speed at whichthe tube is removed from the heating zone. If this speed ratio issmaller than 1, the quartz glass tube is not elongated, but is subjectedto compression. This causes a thickening predominantly on soft and thinwall portions; i.e. on wall portions of a high radial expansion rate.Ideally, the wall thickness over the deformation region is keptapproximately constant and the forming process is thereby stabilized andan exact setting of the outer diameter is enabled.

Therefore, in a preferred method variant, it is intended that theforming process includes at least temporarily a compression phase duringwhich the ratio of a speed at which the hollow cylinder moves into theheating zone and a speed at which the tube is removed from the heatingzone is smaller than 1.

Holders are here welded on the front side to the hollow cylinder to beformed, and these are clamped in a chuck of a glass lathe and rotated insynchronism. A heating source is moved zone by zone along the hollowcylinder. A defined internal pressure can be set in the inner bore ofthe cylinder. Due to the rotation and driven by the centrifugal forceand the internal pressure, the inner bore expands without the chuckhaving to be moved apart for this purpose. The hollow glass cylinder canalso be compressed in the forming process in the direction of therotation axis, wherein the wall thickness of the tube after compressionis between 70% and not more than 100% of the wall thickness prior tocompression. The aim is here a diameter increase in the glass tube whilethe wall thickness thereof is largely maintained. Although a compressingprocess which leads to an increase in the wall thickness (>100%) ispossible, it leads to undesired deformations.

The gas consumption for producing the blow pressure depends on thedegree of the blow pressure. At a high blow pressure it is lower than ata low blow pressure. A high blow pressure is therefore desired. However,when high demands are made on dimensional accuracy and processstability, a procedure is preferred in which the blow pressure is set toless than 20 mbar, preferably to less than 10 mbar.

It has here been found that a high blow pressure can impair the processstability. The tangential tension σT which is operative in the tube wallis mathematically described on the basis of the above equation (1). Thefirst term of this equation which describes the effect of the blowpressure depends on the wall thickness WD of the quartz glass tube. Thethinner the wall, the more significant gets this term. This is shown bythe diagram in FIG. 4. On the ordinate, the tangential tension σ_(T)(N/m²) is plotted against the outer diameter D (m) of a tube of quartzglass to be produced. The starting point is an initial tube with anouter diameter of 197 mm and a wall thickness of 7.5 mm. Curve A showsthe increase in the tangential tension with the end diameter of the tubewhen the blow pressure is substantially used for tube enlargement; i.e.,when the rotational speed is approaching zero. Curve B shows theincrease in the tangential tension with the end tube diameter when thediameter is exclusively enlarged by way of the centrifugal force; i.e.,at a rotational speed of 90 rpm. By comparison, curve C also shows adiameter enlargement just by way of the centrifugal force, but at alower rotational speed of 25 rpm.

It becomes apparent that the tangential tension which is operative inthe wall of the quartz glass tube strongly depends on the blow pressurein the forming process. This has the consequence that wall thicknessdeviations found in the start cylinder are intensified in thedeformation process under the action of blow pressure because a thinnerwall is here subjected to a higher tangential tension than a thickerwall. By comparison, this intensifying effect is smaller in a formingprocess with a purely centrifugal force because the wall thickness hashere no influence on the increase in tangential tension, but just theincreased rotational speed, as shown by the comparison of curve B withcurve C.

With the help of the formerly known forming processes, diametervariations (D₂−D₁) of more than 40 mm were not possible in the formingof hollow cylinders of quartz glass without the acceptance of formingflaws. Such diameter changes can be handled without any problems withthe method according to the present invention. Even at diameter changesof 120 mm in a single forming step, there have been no inhomogeneitiesin the drawn-off tube strand or instabilities in the process sequence.Hence, in the method according to the present invention, great diameterchanges of 70 mm and more and even of 100 mm and more are preferred in asingle forming step.

Hence, the above-mentioned technical objective, starting from a methodof the aforementioned type, is achieved according to the presentinvention also in that Vr,max is smaller than 20 cm/min and that theradial expansion of the softened region is carried out such that thetube is provided with an outer diameter D₂ that is greater by at least70 mm than D₁.

A high stability in the forming process is achievable when a smallradial expansion rate of less than 20 cm/min, preferably less than 10cm/min, is observed. It is thereby possible to set a diameter change of70 mm or more, preferably more than 100 mm, in a single forming step, sothat a particularly economic forming process with a few forming steps ispossible also in the case of great diameter changes. Ideally, only asingle forming step is required. Specifically, it is thereby possible toproduce large tubes of quartz glass with outer diameters of more than500 mm with acceptable energy expenditure and without pronounced drawingstreaks and tolerable siding.

A process variant is preferred in which in the heating zone atemperature is generated that along at least a sub-length of thedeformation zone has a course which is opposite to a profile of thetangential tension along the same sub-length.

Further preferred developments of this procedure correspond to those ashave been described and explained above for the procedure in which therelationship Vr,max<2Va is applicable to the maximum value of thelocation-dependent radial expansion rate Vr, max and the feed rate Va.

FIG. 1 schematically shows an apparatus for forming a hollow cylinder 2of quartz glass into a large tube 22. The forming process comprisesseveral forming steps in which the respective initial hollow cylinder issuccessively formed into the desired large tube 22 with an outerdiameter of 960 mm and a wall thickness of 7.5 mm.

Holding tubes 3 are welded onto the front sides of the hollowquartz-glass cylinder 2 to be formed. The holding tubes 3 are clampedinto a chuck 4 of a horizontal glass lathe 5, and are rotating insynchronism around the rotation axis 6. A burner carriage 21 (see FIG.2), on which several burners are distributed in the form of a ringaround the outer circumference of the hollow cylinder 2, is moved fromone hollow cylinder end to the other end and thereby heats the hollowcylinder 2 of quartz glass zone by zone and over the whole circumferencethereof. The burner carriage 21 is symbolized in FIG. 1 by a dash-dottedcircumferential line 20, illustrating the heating zone. FIG. 2schematically shows a detail thereof. The inner bore 7 of hollowcylinder 2 and large tube 22 can here be flushed with gas through a gasinlet 9, and a defined internal pressure can be set. Driven by thecentrifugal force and the internal pressure, the outer wall of the tubecomes to rest on a molding of graphite 8, which is moved together withthe burner carriage 21.

The burner carriage 21 which moves along the initial hollow cylinder 2from the left to the right, as illustrated by directional arrow 23, isvisible from the detail shown in FIG. 2. Two burner rings 25 a, 25 bthat rotate in parallel around the rotation axis 6 and that serve toheat and soften the initial cylinder 2 are mounted on the burnercarriage 21 one after the other. The two burner rings 25 a, 25 b arespaced apart in axial direction (6) by 50 mm and are adjustable in theirheating output independently of each other. Each of the burner rings 25a, 25 b is formed of five gas burners that are evenly distributed aroundthe longitudinal axis 6 of the cylinder, wherein, viewed incircumferential direction, the individual burners of the burner rows 5a, 25 b are arranged offset to one another.

EXAMPLE

Due to the advance movement of the burner carriage 21 at a speed of 4cm/min, the hollow cylinder 2, while rotating about its longitudinalaxis 6 (which corresponds to the rotation axis) at a speed of 60 rpm, isheated continuously under the action of the burner rings 25 a, 25 b to ahigh temperature of around 2,100° C. In the rear burner ring 25 b, asmaller heating output is generated than in the front burner ring 25 a,resulting in a total heating output density that will be explained inmore detail further below with reference to FIG. 5. To achieve a radialexpansion rate that is as constant as possible along the deformationzone, the axial profile of the heating output curve is here decisive(not so much the absolute value).

The inner bore 7 can here be flushed with a gas, and a defined andcontrolled internal pressure of up to about 100 mbar is here set in theinner bore 7. In the embodiment, a blow pressure of 15 mbar is applied.

The quartz glass is given such a low viscosity through the heating up inthe burner rings 25 a, 25 b that it deforms solely under the action ofcentrifugal force and internal pressure and without use of a moldingtool into the tube 22. The forming process is thus without any tools. Insupport, the outer tube wall comes to rest on a molding 8 of graphite.

For the wall thickness measurement, cameras 26 are arranged in the areaof the start cylinder 2 and in the area of the drawn-off quartz glasstube 22. The cameras 26 are connected to a computer 27 which includes awall thickness control. While the tube strand is rotating, the cameras26 are able to continuously produce a wall thickness profile that isevaluated in the computer 27, such that the amount of wall one-sidedness(maximum value minus minimum value of the wall thickness) and thecircumferential position of the minimum wall thickness and the maximumwall thickness over the outer circumference are determined.

In the forming process, an additional elongation does not take placeautomatically. The start cylinder is often even compressed such that theinflated quartz glass tube 22 has about the same wall thickness as thehollow cylinder 2. In the present embodiment, there is a compression of15%; i.e., the cross-sectional area of the tube 22 is 15% greater thanthe cross-sectional area of the hollow cylinder 2.

The radial expansion rate is determined on the basis of the aboveequations (1) to (3); it is set such that even in the maximum it islower than 8 cm/min. For the density of quartz glass, the value 2200kg/m³ is used, and for the viscosity η of quartz glass, there is thefollowing equation:

${\lg\mspace{11mu}\eta} = {1.6 + {\frac{8487}{{T\left\lbrack {{^\circ}\mspace{11mu}{C.}} \right\rbrack} - 390}{Pas}}}$

The quartz glass tube 22 obtained in this way can serve as an initialhollow cylinder for a further forming process. The original hollowcylinder 2 is thereby enlarged in steps into a large tube of quartzglass, wherein each forming step represents a diameter enlargement of120 mm. The outer diameter of the burner rings 25 a, 25 b and the workdistance of the molding tool 8 are here adapted to the respective outerdiameter of the forming step.

This method yields a large tube of synthetic quartz glass or of quartzglass of naturally occurring raw material with an altogether highdimensional stability in an economic way. The wall thickness variationof the large tube of quartz glass produced thereby is less than 1 mm pertube length meter.

The diagram of FIG. 4 demonstrates the influence of the blow pressure onwall thickness variations. On the ordinate, the tangential tension σ(N/m²) is plotted against the outer diameter D (m) of the quartz glasstube to be produced. An initial tube with an outer diameter of 197 mmand with a wall thickness of 7.5 mm is started from. Curve A shows theincrease in the tangential tension with the end diameter of the tube ifthe blow pressure is largely used for tube enlargement; i.e., if therotational speed is approaching zero. Curve B shows the increase in thetangential tension with the end tube diameter when the diameter isexclusively enlarged by way of the centrifugal force; i.e., at arotational speed of 90 rpm. By comparison, curve C also shows a diameterenlargement just by way of the centrifugal force, but at a lowerrotational speed of 25 rpm.

It is evident that the tangential tension which is operative in the wallof the quartz glass tube considerably depends on the blow pressure inthe forming process. This has the effect that wall thickness deviationsfound in the start cylinder are intensified in the deformation processunder the action of blow pressure because a thinner wall is here exposedto a higher tangential tension than a thicker wall. By comparison, thisintensifying effect turns out to be smaller in a forming process with apurely centrifugal force because the wall thickness has here noinfluence on the increase in the tangential tension, but just theincreased rotational speed, as is shown by the comparison of curve Bwith curve C.

FIG. 5 shows a diagram demonstrating the influence of the temperature inthe area of the deformation zone on the geometric design thereof. On theleft ordinate, the radius R in [m] of the “shoulder” is plotted againstthe longitudinal axis position x in [mm], and the power density L in(W/m] on the right ordinate.

The diagram shows two pairs of curves by way of comparison. In the pairof curves L1/R1, the radius course R1 of the shoulder was determined onthe basis of a formerly usual distribution of the power density L1. Thepair of curves L2/R2 shows the effect of an adaptation of thedistribution of the power density L2 according to the present inventionon the radius course R2 of the shoulder.

It must here be noted that the profile of the power input is notidentical to the temperature profile because the heat in the glass isfurther transported and is, so to speak, added up. That is why the powerinput always ends approximately on the turning point of the shoulder;i.e., at the steepest point thereof. The illustrated power profile L2 isobtained when the associated temperature profile is adapted such thatthe radial deformation rate within the deformation zone is approximatelyconstant. This adaptation leads to a broader distribution of the powerdensity.

The adaptation in the pair of curves L2/R2 refers to the axialtemperature distribution, and it aims at compensating the tangentialtension changing along the deformation zone (30) in such a manner that aradial expansion rate that is as constant as possible is obtained alongthe entire deformation zone (30). At any rate, the radial expansion ratedoes not exceed the above-mentioned maximum value of 8 cm/min.

The calculation of the corresponding axial temperature profile iscarried out on the basis of the above-indicated equation (4). It isthereby ensured that the temperature profile at an axial position alongthe deformation zone has a minimum at which the profile of thetangential tension has a maximum.

Moreover, it has been found that the comparatively broader distributionof the power density L2, despite a much lower maximum value (about230.00 W/m in comparison with about 260.00 W/m at L1), effects a muchgreater end radius at R2 (about 0.160 m in comparison with about 0.125 mat R1). It is also visible that the slope within the shoulder contour ofR2 is approximately constant over a long distance, and that despite thelarger end radius, the maximum slope hardly differs from that of R1. Theconstant slope of the shoulder is a sign of a stable forming process.

Comparative Example

In another forming process, rotational speed, heating temperature andtemperature profile are set according to the above-explained embodiment,but the feed rate Va of the burner carriage 11 is reduced to a rate ofabout 3 cm/min.

Due to the reduced feed rate, a comparatively higher radial expansionrate Vr is obtained in the deformation zone 20. A maximum value Vr,maxabove about 6 cm/min is obtained on the basis of equations (1) to (3).

The original hollow cylinder 2 is expanded in steps into a large tube ofquartz glass, wherein each forming step represents a diameterenlargement of 120 mm. The wall thickness variation of the large tube ofquartz glass produced thereby is more than 1 mm per tube length meterand is inadequate for an application where high demands are made on thedimensional stability.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

We claim:
 1. A method for producing a tube (22) of glass by forming ahollow cylinder (2) from the glass with a first outer diameter D₁ inthat said cylinder, while rotating about a rotation axis (6), issoftened in portions in a heating zone (20) which is moved at a relativefeed rate Va, and the softened portion is radially expanded under anaction of a centrifugal force and/or of an internal overpressure appliedin a bore (7) of the hollow cylinder (2) so as to form a deformationzone (30), and the tube (22) is continuously shaped with a second outerdiameter D₂ which is greater than the first outer diameter D₁, whereinthe radial expansion of the softened portion is carried out at alocation-dependent radial expansion rate Vr, a profile of which alongthe deformation zone has a maximum value Vr,max which is smaller than 2times the relative feed rate Va.
 2. The method according to claim 1,wherein the maximum value Vr,max of the radial expansion rate is smallerthan 1.5 times and particularly preferably smaller than 0.7 time, therelative feed rate Va.
 3. The method according to claim 1, wherein, inthe heating zone (20), a temperature profile is generated that along atleast a sub-length of the deformation zone (30) has a course opposite toa profile of a tangential tension along the same sub-length.
 4. Themethod according to claim 1, wherein a temperature at an end of thedeformation zone (30) is lower than a temperature at a beginning of thedeformation zone.
 5. The method according to claim 3, wherein theheating zone (20) has a front heating portion assigned to the beginningof the deformation zone (30) and a rear heating portion assigned to theend of the deformation zone (30), wherein a higher temperature isproduced by the front heating portion on a surface of the deformationzone (30) than by the rear heating portion.
 6. The method according toclaim 1, further comprising at least one compression phase during whicha ratio of a speed at which the hollow cylinder (2) moves into theheating zone (20) and of a speed at which the tube (22) is removed fromthe heating zone (20) is smaller than
 1. 7. The method according toclaim 1, wherein the maximum radial expansion rate Vr,max is smallerthan 20 cm/min, preferably smaller than 10 cm/min.
 8. The methodaccording to claim 1, wherein a blow pressure is set to less than 20mbar, preferably to less than 10 mbar.
 9. The method according to claim1, wherein the tube (22) is produced with the second outer diameter D₂which is greater than the first outer diameter D₁ by at least 40 mm.